ROOT POROSITIES AND RADIAL OXYGEN LOSSES OF RUMEX AND PLANTAGO SPECIES AS INFLUENCED BY SOIL PORE DIAMETER AND SOIL AERATION

The effects of hypoxia in ballotini and quartz sand with respectively large and small soil pore diameters on root porosity was studied for R umex palustris Sm., Rumex acetosa L. and Plantago major L. ssp. major. Under hypoxic conditions R. palustris produced large root pores when large soil pores were present. Absence of such large soil pores result ed in the collapse of the root structure and a reduced root growth. In creases in intercellular spaces in P. major roots seemed to result in a higher root porosity upon hypoxia, a small soil pore diameter or a c ombination of both but this was not significant. Only a limited number of roots with large root pores was produced. The morphological struct ure of P. major roots without large root pores remained intact also in soils with a small soil pore diameter. R. acetosa only slightly incre ased its root porosity upon hypoxia and its morphological structure al so remained intact when soil pore diameter was small. Radial oxygen lo ss (ROL) was found in R. palustris roots when grown in waterlogged, un compacted soils. P. major had some roots with ROL in all treatments ex cept in waterlogged, compacted soil. R. acetosa did not produce any ro ots showing ROL. Under field conditions aerenchyma is of no use in wat er saturated, compacted soils. This explains why a species like R. pal ustris only grows on hypoxic soil when it is not compacted, in contras t to P. major which will grow on moist, compacted soils. R. acetosa ca nnot be found on either very moist or heavily compacted soil, since it does not produce a healthy root system under these conditions.